This invention relates to a superheterodyne receiver for receiving an amplitude-modulated RF signal.
To generate and demodulate single-sideband signals, three systems have been developed: a filter method, a phasing method, and a so-called "Third Method". The latter has many uses beyond its original application. Generally speaking, it permits any frequency band to be arbitrarily shifted in frequency, all adjacent frequencies and mixer products being simultaneously suppressed by simple means. The only filtering devices required are low-pass filters which not only eliminate unwanted mixer products but also suppress interferences caused by frequencies outside the original frequency band. The main disadvantage of this method is due to the fact that the frequency band to be shifted simultaneously passes through two branches which should be of exactly the same design but are not in practice.
While being processed, the frequency band is imaged onto itself, but the image components disappear again if the two branches are exactly alike. If the branches are unlike, interfering components will remain which, however, originate from the frequency band to be processed, not from an external frequency band. In a speech or music signal, for example, a low-pitched tone, for example causes a highly attenuated high-pitched tone, whose amplitude correlates with that of the low-pitched tone. If, in addition, a pilot signal or a subcarrier is present whose amplitude does not depend on that of the speech or music signal or even rises when the speech or music signal disappears, the "image" of the pilot signal or subcarrier will, in most cases, cause spurious unwanted responses.
The demodulation of a single-sideband signal is nothing but a frequency shift from the RF range to a baseband. The generation of a single-sideband signal is nothing but a shift of the frequency of a baseband signal to the RF range. A r eversal of the entire frequency band may be necessary, but this is mathematically equivalent to a shift to the negative frequency range, so that besides the choice of different oscillator frequencies, no additional steps have to be taken. The demodulation of a vestigial-sideband-modulated television signal, too, is nothing but a frequency shift to the baseband, so that the original picture carrier drops to zero frequency. If the television signal is demodulated by the "Third Method", the image of the picture carrier falls within the video signal, and thus, results in a spurious pattern (moire) in the reproduced picture. In a color television receiver, hue errors additionally result from the interaction of the images of the picture carrier and the chrominance subcarrier.